1. Field of the Invention
This invention in general refers to dispersion management within a laser cavity and more particularly to a laser apparatus where dispersion management is used to enhance the performance of the laser by reducing the timing jitter of the pulses produced by the laser and by reducing the occurrence of pulse dropouts or multiple pulse production.
2. Description of the Related Art
Dispersion management within a laser cavity allows a fiber laser to produce optical pulses of lower amplitude and phase noise and with greater immunity to pulse dropouts than is otherwise possible. These features are of great importance to lasers used as sources in telecommunications applications, as research instruments, or in optical-to-digital conversion and analysis application.
Dispersion management is a concept usually encountered only in optical soliton fiber transmission applications. In that field, it has been found that the use of lengths of at least two types of fibers with different chromatic dispersions, usually of differing signs, but with a (usually small) net anomalous dispersion, yields a transmission medium with distinct advantages over one with a uniform dispersion: i) the optical energy of the solitons carrying information is greater than those in a uniform-dispersion fiber with the net dispersion, yielding a greater signal-to-noise (S/N) ratio of the signal; ii) the timing jitter in the system is lower; iii) the pulses tend to have a Gaussian temporal profile, reducing pulse-to-pulse interactions; iv) nonlinear pulse interactions such as four-wave mixing are strongly reduced because of the strong local dispersion. The strength of dispersion management is usually expressed as a unitless parameterγ=2Σn|β″nln|/τ2 where β″n and ln are the dispersion and length respectively, of fiber segment n, and τ is the pulse duration.
Dispersion-managed solitons have two salient properties for the purposes herein: their energy is greater than that of an equivalent uniform-dispersion soliton, and their pulse duration changes much less with a change in pulse energy than does an equivalent uniform-dispersion soliton.
In an actively harmonically mode-locked soliton laser four regimes of operation are expected to be seen as the amount of energy per pulse is varied: (i) At the lowest energies, the laser cannot form solitons, and a very noisy output of long-duration pulses is evident. (ii) At somewhat higher energies, there is not enough power available for a full train of solitons, so the laser can form either a train of long-duration pulses or an occasional soliton. The laser loss is higher for long-duration pulses, since they would be clipped by the finite duration of the amplitude mode-locking time window. Therefore a combination of solitons and dropouts is observed. (iii) At higher frequencies, the laser can produce an uninterrupted stream of solitons. In this regime, the pulse duration becomes briefer as the pulse energy increases. (iv) At the highest energies, pulses are prevented from becoming more brief by the fact that energy is lost when they become so brief and their bandwidth consequently becomes so large that the pulses are clipped spectrally by the finite gain bandwidth (or the bandwidth of an intracavity bandpass filter). Then multiple pulses begin to appear in each time slot.
The desirable operating regime from a telecommunications standpoint is (iii), in which an uninterrupted stream of pulses is generated. The maximum and minimum pulse widths which define the boundaries of this regime are determined by the details of the laser parameters.
In a laser of uniform dispersion, the pulse energy range over which the laser operates in the stable regime can be as small as 30%. One characteristic of a dispersion-managed laser is that the pulse duration decreases by comparatively little as the pulse energy increases; as a consequence, the laser can operate in the stable regime over a pulse energy range of as much as a factor of 100.